Solar-powered refrigeration system

ABSTRACT

A solar powered vapor compression refrigeration system is made practicable with thermal storage and novel control techniques. In one embodiment, the refrigeration system includes a photovoltaic panel, a variable speed compressor, an insulated enclosure, and a thermal reservoir. The photovoltaic (PV) panel converts sunlight into DC (direct current) electrical power. The DC electrical power drives a compressor that circulates refrigerant through a vapor compression refrigeration loop to extract heat from the insulated enclosure. The thermal reservoir is situated inside the insulated enclosure and includes a phase change material. As heat is extracted from the insulated enclosure, the phase change material is frozen, and thereafter is able to act as a heat sink to maintain the temperature of the insulated enclosure in the absence of sunlight. The conversion of solar power into stored thermal energy is optimized by a compressor control method that effectively maximizes the compressor&#39;s usage of available energy. A capacitor is provided to smooth the power voltage and to provide additional current during compressor start-up. A controller monitors the rate of change of the smoothed power voltage to determine if the compressor is operating below or above the available power maximum, and adjusts the compressor speed accordingly. In this manner, the compressor operation is adjusted to convert substantially all available solar power into stored thermal energy.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] This invention relates generally to solar power control systems,and in particular, to an efficient system and method for applyingsolar-generated power to refrigeration.

[0003] 2. Description of the Related Art

[0004] Two billion people live without electricity. They represent amarket for various solar powered systems such as stand-alone powersystems and small capacity solar refrigerators. Efforts have been madeto develop stand-alone photovoltaic (PV) power systems that providelighting and power for small devices such as radios and smalltelevisions. For example, such systems may include a solar panel, abattery, and a low wattage fluorescent light. Solar refrigerators,however, represent a bigger challenge.

[0005] Previous attempts to produce a marketable solar refrigerator havebeen largely unsuccessful. For example, consider the following patents:

[0006] In U.S. Pat. No. 4,126,014, Thomas Kay discloses an absorptionrefrigeration system powered by a heated fluid from a solar panel.

[0007] In U.S. Pat. No. 5,501,083, Tae Kim discloses an AC-powered airconditioner having a solar panel for backup electrical power.

[0008] In U.S. Pat. No. 5,497,629, Alexander Rafalovich discloses theuse of solar power in an air conditioning system to pump heat from anindoor space to a thermal store.

[0009] In U.S. Pat. No. 5,685,152, Jeffrey Sterling discloses using aheated medium from solar collectors to produce a cold thermal store andmechanical energy to pump heat from an indoor space to the cold thermalstore.

[0010] Kay's refrigeration system provides no means to maintainrefrigerator operation in the absence of sunlight (e.g. at nighttime oron overcast days). As the air conditioning systems are largely unsuitedfor even small capacity refrigerators or freezers, no attempt has beenmade to scale these systems to produce a commercializable solarrefrigerator.

[0011] Accordingly, it is desirable to provide an efficient,inexpensive, commercializable small capacity solar refrigerator whichcan operate for several days in the absence of sunlight. As batteriesare often expensive and require regular maintenance, it would further bedesirable to provide such a solar refrigerator which does not requirebatteries.

SUMMARY OF THE INVENTION

[0012] A solar powered vapor compression refrigeration system is madepracticable with thermal storage and novel control techniques. In oneembodiment, the refrigeration system includes a photovoltaic panel, acapacitor, a compressor, an insulated enclosure, and a thermalreservoir. The photovoltaic (PV) panel converts sunlight into DC (directcurrent) electrical power, some of which is stored in the capacitor. Thecapacitor provides additional current during compressor start-up, andthereafter acts to smooth out variations in the power voltage. The powerfrom the PV panel and capacitor drives the compressor to circulaterefrigerant through a vapor compression refrigeration loop, therebyextracting heat from the insulated enclosure. The thermal reservoir issituated inside the insulated enclosure and includes a phase changematerial. As heat is extracted from the insulated enclosure, the phasechange material is frozen. Thereafter the thermal reservoir is able toact as a heat sink to maintain the temperature of the insulatedenclosure for an extended period in the absence of sunlight.

[0013] This conversion of solar power into stored thermal energy isoptimized by a compressor control method that effectively maximizes thecompressor's usage of available energy. A controller monitors the rateof change of the smoothed power voltage to determine if the compressoris operating below or above the maximum available power, and adjusts thecompressor speed accordingly. In this manner, the compressor operationis continuously adjusted to convert substantially all available solarpower into stored thermal energy.

BRIEF DESCRIPTION OF THE DRAWINGS

[0014] A better understanding of the present invention can be obtainedwhen the following detailed description of preferred embodiments isconsidered in conjunction with the following drawings, in which:

[0015]FIG. 1 is a block diagram of a first solar refrigeration systemembodiment;

[0016]FIG. 2 is a block diagram of a second solar refrigeration systemembodiment;

[0017]FIG. 3 is a graph of an exemplary I-V curve for a photovoltaicpanel;

[0018]FIG. 4 is a graph of an exemplary I-V curve for a photovoltaicpanel in reduced light;

[0019]FIG. 5 is a flowchart of a first compressor speed control method;and

[0020]FIG. 6 is a flowchart of a second compressor speed control method.

[0021] While the invention is susceptible to various modifications andalternative forms, specific embodiments thereof are shown by way ofexample in the drawings and will herein be described in detail. Itshould be understood, however, that the drawings and detaileddescription thereto are not intended to limit the invention to theparticular form disclosed, but on the contrary, the intention is tocover all modifications, equivalents and alternatives falling within thespirit and scope of the present invention as defined by the appendedclaims.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

[0022] Turning now to the figures, FIG. 1 shows a first embodiment of asolar refrigeration system which includes a solar panel 102 connected toa power bus 103. Although a wide variety of solar panel types and stylesmay be employed, one suitable example is a 12 volt nominal PV panel thatis capable of a peak power output of approximately 120 watts atapproximately 15 volts under full solar insolation.

[0023] A capacitor 104 is connected to power bus 103 in parallel withsolar panel 102. Capacitor 104 operates to provide temporary storage ofelectrical charge in order to smooth any voltage variations on power bus103 and to provide extra current during demand periods. The voltagevariations may be caused by a variety of sources including changes inlight intensity on the solar panel and changes in the electrical loaddriven by the solar panel 102. The capacitor 104 may be varied in sizeand type, but a preferred example is a 0.2 Farad electrolytic capacitor.

[0024] A variable speed compressor 108 with a load controller 106 isdirectly coupled to the solar panel 102 by power bus 103. In thiscontext, “directly coupled” is defined to mean that no power convertersare provided between the compressor 108 and solar panel 102. Althoughother embodiments are also contemplated, this embodiment advantageouslyexhibits relatively high efficiency due to the direct powering of thecompressor 108 by a PV panel. It is noted that systems which usebatteries typically force the solar panel to operate below its peakpower point to match the battery charging voltage. Powering thecompressor directly from the solar panel allows the solar panel to beoperated at the maximum power point.

[0025] The variable speed compressor 108 is preferably a direct currentcompressor such as a Danfoss® BD35F direct current compressor withrefrigerant 134 a. Persons of skill in the art will recognize that othersuitable compressors and refrigerants can be employed. The BD35Fincludes a “brushless” DC (direct current) motor in that providespermanent magnets on the rotor. Electronics in the BD35F switch the DCinput to provide a 3-phase input to fixed coils that drive the rotor.The electronics improve the motor's efficiency by sensing the back-EMFin the coils to determine the rotor position. This compressorimplementation is believed to exhibit efficiency and longevityadvantages over typical DC compressors. As discussed in further detailbelow, load controller 106 senses the voltage on power bus 103 andregulates the speed of compressor 108 in response to variations in thisvoltage.

[0026] Compressor 108 circulates refrigerant through a vapor compressionrefrigeration loop that preferably includes a first heat exchanger(a.k.a. a condenser) 110, a capillary tube 112, a second heat exchanger(a.k.a an evaporator) 114 internal to an insulated enclosure 120, and athird heat exchanger (sometimes referred to as SLLL HX, or the suctionline/liquid line heat exchanger) 116 associated with the capillary tube112. As refrigerant is circulated through the loop, it is compressed bycompressor 108, cooled to a liquid state by ambient air in condenser110, flash-cooled by heat exchanger 116 in capillary tube 112,evaporated to a gaseous state in evaporator 114, warmed by heatexchanger 116, and recompressed and re-circulated by compressor 108.This circulation results in a net transfer of heat from the evaporator114 to the condenser 110, thereby cooling the interior of the insulatedenclosure 120 by heating ambient air. One of skill in the art willreadily recognize that this refrigerant loop may be constructed invarious suitable manners, and that other refrigerant loops may also beemployed to achieve a net transfer of heat energy away from theinsulated enclosure 120 without departing from the scope of theinvention. For example, one specific alternate implementation uses anexpansion valve in place of the capillary tube 112.

[0027] Similarly, many types of insulated enclosures are well known andmay be employed, but a preferred construction for the insulatedenclosure 120 uses fiberglass-reinforced plastic shells for the cabinetwith vacuum panels between the inner and outer shells for insulation. Abezel interface is preferably provided between the cabinet and the doorto minimize thermal conductance and convection through the seal. Withthis preferred construction, a cabinet having a composite R value(thermal resistance in units of hr·ft²·° F./BTU) of 26 has beenachieved. (Most conventional refrigerators have a composite R value of5.)

[0028] Referring still to FIG. 1, the load controller 106 senses thevoltage on power bus 103 and provides a speed control signal 107 tovariable speed compressor 108. By controlling the compressor speed, theload controller 106 effectively maximizes the power extracted from thesolar panel. It inexpensively implements an advantageous optimizationmethod as described in further detail below. While it can take variousforms, the load controller 106 is preferably implemented in the form ofa microcontroller that implements a software algorithm. Themicrocontroller may also be designed to perform other system functionssuch as: monitoring internal temperature of the insulated enclosure,monitoring the compressor for error conditions, and initiatingcompressor start-ups and shut-downs in a manner designed to extend thelife of the compressor. In alternate embodiments, the load controller106 may also control power source switching to access alternate powersources, if available and when necessary, or to provide redundancy (inthe case of multiple solar panels).

[0029] A thermal reservoir 118 is preferably provided in the insulatedenclosure 120. Thermal reservoir 118 preferably comprises a phase-changematerial that has a phase-change temperature at or slightly below thetarget interior temperature for the insulated enclosure. Particularlydesirable phase-change materials are those having a solid-liquid phasechange with a high heat of fusion, and which are inexpensive andrelatively non-toxic. Water and water solutions are examples of suitablephase change materials. A water solution of approximately 3-5% propyleneglycol may be particularly desirable, as it exhibits a reduced tendencyto rupture closed containers when freezing. The size and phase changematerial of the thermal reservoir is preferably chosen to maintain thetarget interior temperature for several days in the absence of solarpower (or at least 36 hours). One of skill in the art will recognizethat thermal reservoir 118 may be implemented in a variety of suitableconfigurations.

[0030] In the embodiment of FIG. 1, the thermal reservoir 118 iscontemplated as being adjacent to evaporator 114, and/or as being a partof evaporator 114. As refrigerant circulates through the evaporator 114to cool the interior of the insulated enclosure 120, a direct transferof heat energy occurs to evaporator 114 from thermal reservoir 118 tocool the thermal reservoir and induce a phase change of the phase-changematerial. In other words, if the phase-change material is water, theflow of refrigerant through the evaporator cools and freezes the water.

[0031] In operation, the solar panel 102 delivers power to power bus 103during the day when the sun is shining. The load controller 106 runs thecompressor 108 at a speed that maximizes the power extracted from thesolar panel. The compressor 108 circulates refrigerant through arefrigerant loop to cool the insulated enclosure and to cool and inducea phase change of the material in the thermal reservoir. At night andduring adverse weather conditions, no power is delivered to the powerbus 103, and the compressor 108 is inactive. The temperature in theinsulated enclosure is maintained by the thawing of the material in thethermal reservoir. Advantageously, no fluid circulation or active heatpumping is required to maintain the enclosure temperature during theseinactive time periods.

[0032] Referring now to FIG. 2, a second solar refrigeration systemembodiment is shown. In this embodiment, an alternate power source 205is coupled to power bus 103. The alternate power source 205 may takemany forms including, e.g. a supplemental battery, a fuel cell, agenerator, or an AC/DC converter connected to a commercial AC powergrid. The load controller 106 turns the alternate power source 205 on oroff by means of an enable signal 206. The load controller 106 preferablyminimizes the use of alternate power source 206 to the greatest extentpossible, using it only when solar power is unavailable and thetemperature of the insulated enclosure exceeds a predeterminedthreshold. The load controller 106 monitors the interior temperature ofinsulated enclosure 120 by means of a temperature signal 207 from atemperature sensor (not shown) in insulated enclosure 120.

[0033] The solar refrigeration system embodiment of FIG. 2 also employsan alternate configuration for the evaporator 114 and thermal reservoir118. In this configuration, the refrigerant passing through evaporator114 cools a second fluid that is pumped through the evaporator 114 by apump 209. Many fluids may be used, but currently a propylene glycol andwater mixture is preferred. The cooled second fluid is then circulatedthrough a heat exchanger in the thermal reservoir 118 to cool and inducea phase change in the phase change material. The load controller 208 maybe configured to turn pump 209 on and off by means of a signal 208. Pump209 is preferably activated only when compressor 108 is operating. A fanmay be provided to improve air circulation, and may also be controlledby signal 208.

[0034] In one particular implementation of the alternate configurationshown by FIG. 2, the cooling of the insulated enclosure 120 isaccomplished primarily by the thermal reservoir 118 and the heatexchanger therein. This implementation may prove advantageous relativeto the configuration shown in FIG. 1 for several reasons. A firstfeature of this implementation is that the refrigerant volume isreduced, which may provide reduced cost and increased system longevity.A second feature of this implementation is that thermal leakage to theinterior of the insulated enclosure during and after compressorshut-down is reduced. A third feature is that mechanical design of thethermal reservoir may be simplified due to a larger and more favorablydistributed heat exchange area with the phase change material. It isnoted that the solar refrigeration system embodiment of FIG. 1 may bemodified to use this thermal reservoir configuration.

[0035] The load controller 106 may be designed to monitor thetemperature of the insulated enclosure and respond to temperatureexcursions above or below predetermined thresholds. As mentionedpreviously, the load controller 106 may activate alternate power source205 in response to a detected temperature above an upper temperaturelimit. Also, the load controller 106 may halt the variable speedcompressor 108 in response to a detected temperature below a lowertemperature limit. Once the temperature returns to the desired range,the load controller 106 may then resume normal solar-powered operation.One of skill in the art will recognize the desirability of providingsome hysteresis in any such temperature regulation strategy. It is notedthat the upper temperature limit is preferably slightly above the phasechange temperature, and the lower temperature limit is preferablyslightly below the phase change temperature.

[0036] As previously mentioned, load controller 106 operates to maximizethe power drawn from the solar panel 102. Various methods which may beimplemented by the load controller are now described with reference toFIGS. 3 and 4. FIG. 3 shows an I-V curve 302 representing the voltage Vprovided by solar panel 102 as a function of current I drawn from thesolar panel, assuming maximum insolation (sunlight intensity). Thevoltage varies from V_(OC) when no current is drawn to 0 when the shortcircuit current I_(SC) is drawn. A typical example of an open circuitvoltage V_(OC) for a nominal 12 volt panel is 20 volts, and a typicalexample of a short circuit current is 8 amperes. On the curve betweenthese two points is a maximum power point (I_(MP),V_(MP)) where themaximum power is extracted from the solar panel. This point occurs wherethe slope of the curve is dV/dI=−V/I.

[0037] The load controller 106 preferably locates this maximum powerpoint by an iterative search process. At an initial time t=0, thecompressor 108 is not running, and no current is drawn. The loadcontroller determines that a sufficient start-up voltage exists andstarts the compressor at a minimum startup speed. Note that the currentdrawn by the compressor increases as the speed of the compressorincreases. At a subsequent time t=1, the compressor is drawing a currentand the voltage provided by the solar panel has been slightly reduced.The load controller 106 then begins gradually increasing the speed ofthe compressor 108, detecting the power bus voltage at regular intervalsand adjusting the speed of the compressor in response to some criterionbased on the detected voltage. The time progression of operating pointshas been exaggerated for illustration. In a preferred embodiment, theincrements in speed are digital and are much smaller, so that 255 ormore operating points on the curve are possible.

[0038] Various adjustment criteria may be used. For example, referringmomentarily to FIG. 4, a second I-V curve 402 is shown for reducedinsolation. The maximum power point on curve 402 has shifted relative tothe maximum power point on curve 302. It is noted that while the currentI_(MP) at the maximum power point is particularly sensitive to theamount of insolation, the voltage V_(MP) at the maximum power point isrelatively insensitive to the amount of insolation. Consequently, theload controller 106 may increase or decrease the compressor speed asneeded to maintain the power bus voltage close to a predeterminedvoltage target, e.g. the maximum power voltage for full solarinsolation.

[0039] While simple, this criterion is suboptimal since the maximumpower voltage varies with temperature, and in any case, this criteriondoes not provide for full power extraction during reduced insolation.Referring again to FIG. 3, it is noted that at all operating pointvoltages on curve 302 above the maximum power point voltage, the powerprovided by the solar panel increases as the current increases, whereasfor all operating point voltages on the curve below the maximum powerpoint voltage, the power provided by the solar panel DECREASES as thecurrent increases. When this observation is combined with theobservation that the power required by the compressor always increasesas the speed increases, an improved control method can be developed forthe load controller 106.

[0040] Referring simultaneously to FIGS. 1 and 3, it is noted that whenthe compressor 108 is run at a speed requiring less power than the solarpanel 102 can provide, an increase in compressor speed will result in amatching increase in power extracted from the solar panel. Due to thecapacitor 104, the power bus voltage will decrease smoothly andstabilize. In other words, the magnitude of the time derivative of thevoltage decreases as a function of time. When the compressor 108 is runat a speed requiring more power than the solar panel 102 can provide,the charge on capacitor 104 provides the extra power required. Sinceonly a limited amount of charge exists on capacitor 104, the capacitor104 is increasingly depleted as time goes on, and the compressorattempts to draw more current from solar panel 102. This in turn causesthe solar panel to provide less power as the voltage drops, causingfurther depletion of the capacitor and even more current draw from thesolar panel 102. The power bus voltage rapidly decays, and in fact, therate of voltage decay increases as a function of time. Expressed incalculus terms, when the second derivative of the voltage with respectto time is greater than or equal to zero, the system is operating on thecurve above the maximum power point voltage. When the second derivativeof the voltage with respect to time is less than zero, the system isoperating on the curve below the maximum power point voltage.

[0041]FIG. 5 shows a first improved control method which may beimplemented by load controller 106. After the load controller hasstarted the compressor and allowed some small amount of time for thevoltage on the power bus to settle into a steady state, the loadcontroller begins sampling the voltage at regularly spaced timeintervals. One of skill in the art will recognize that the samplingintervals may be allowed to vary if this is determined to be desirable,and appropriate adjustments can be made to the method. Additionally, thepower bus voltage signal may be mildly conditioned to remove highfrequency noise before being sampled by the load controller.

[0042] In step 502 an initial voltage sample is taken before the loadcontroller enters a loop consisting of steps 504-516. For each iterationof the loop, two additional voltage samples are taken. In step 504, afirst voltage sample is taken, and in step 506 a first change in thevoltage is calculated by subtracting the previous voltage sample fromthe first voltage sample. In step 508, a second voltage sample is taken,and in step 510 a second voltage change is calculated by subtracting thefirst voltage sample from the second voltage sample.

[0043] In step 512, the two calculated voltage changes are compared. Ifthe magnitude of the second voltage change is less than or equal to themagnitude of the first voltage change, then in step 514, the loopcontroller increments the speed of the compressor by one step. On theother hand, if the magnitude of the second voltage change is larger thanthe magnitude of the second voltage change, then in step 516, the loopcontroller decrements the speed of the compressor by two or more steps.While various implementations of decrement step 516 are contemplated, itis currently preferred to make the number of decrement steps apredetermined constant based on the system embodiment. It is furthercontemplated to make the increment step sizes adaptive in nature. Theadaptation may be based on the size of the calculated first voltagechange, so that smaller voltage changes result in smaller step sizes. Inthis manner, the load controller may more quickly and accurately locatethe maximum power point. The nature of the adaptation may be changedafter the first time the speed is decremented to provide for a smallerrange of variation about the optimal operating point. For example, thestep size may become based proportionally on the size of the secondcalculated voltage change, so that larger voltage changes result inlarger step sizes.

[0044]FIG. 6 shows a second improved control method which may beimplemented by load controller 106. When the system is operating on theportion of the solar panel curve below the maximum power point, thecalculated voltage changes continually grow if the compressor speed isnot adjusted. Hence the method of FIG. 5 may be simplified byeliminating steps 508 and 510, and replacing step 512 with step 612, inwhich the calculated voltage change is compared with a predeterminedthreshold. No matter where the system is operating on the lower part ofthe curve, eventually the calculated voltage change will exceed thethreshold, and the compressor speed will be reduced accordingly. Whenthe voltage change is less than the threshold, the system is assumed tobe on the upper part of the curve, and the compressor speed isincreased. The threshold is preferably adjusted to allow for only asmall range of variation around the maximum power point.

[0045] It is noted that the disclosed refrigeration systems and powercontrol methods may have many varied embodiments. For example, onerefrigeration system embodiment may employ an insulated enclosure withdivided compartments that are maintained at different temperatures suchas might be suitable for storing fresh and frozen foods. Anotherembodiment may employ the structure and stored contents of the insulatedenclosure as the thermal reservoir. This latter approach may proveparticularly suitable for refrigeration systems that are configured toproduce the stored contents, such as would be the case for an ice maker.Some embodiments may include alternate energy sources such as batteries,a generator, or a commercial power grid, the use of which is may beminimized by using the solar panel as much as possible. Theseembodiments could use a smaller thermal reservoir due to availability ofan alternate power source to maintain the temperature. In someembodiments, the refrigeration system may be applied to cool poorlyinsulated enclosures that are often exposed to substantial amounts ofsunlight. In this vein, one refrigeration embodiment is an airconditioning system for vehicles that cools the interior when thevehicle is exposed to the sun. Such a system may or may not include someform of phase change material as a thermal reservoir.

[0046] Numerous such variations and modifications will become apparentto those skilled in the art once the above disclosure is fullyappreciated. It is intended that the following claims be interpreted toembrace all such variations and modifications.

What is claimed is:
 1. A refrigeration system which comprises: anenclosure having an interior space; phase-change material situated insaid interior space; a photovoltaic panel configured to provideelectrical power having a DC voltage; and a compressor electricallyconnected to the photovoltaic panel to receive electrical power at saidDC voltage, wherein the compressor is configured to circulaterefrigerant through a first heat exchanger to cool the refrigerant,through a constriction configured to sustain a pressure drop, andthrough a second heat exchanger to extract heat from said interiorspace.
 2. The refrigeration system of claim 1, further comprising: apump configured to circulate a fluid through the second heat exchangerto cool the fluid, and through a third heat exchanger to extract heatfrom the phase change material.
 3. The refrigeration system of claim 1,wherein at least a portion of the second heat exchanger is situatedadjacent to the phase change material.
 4. The refrigeration system ofclaim 1, wherein the phase-change material mass, when solidified, issufficient to maintain the interior space of the enclosure substantiallyat a phase-change temperature for more than 36 hours.
 5. Therefrigeration system of claim 1, wherein the phase-change materialcomprises water.
 6. The refrigeration system of claim 1, furthercomprising a capacitor coupled to the compressor to smooth variations insaid DC voltage.
 7. The refrigeration system of claim 6, wherein thecompressor is a variable speed compressor, and wherein the refrigerationsystem further comprises a controller configured to monitor the DCvoltage and to regulate the compressor speed to run the compressor at asubstantially maximum available power.
 8. The refrigeration system ofclaim 7, wherein the controller is configured to receive a temperaturesignal indicative of a temperature of the interior space of theenclosure, and wherein the controller is configured to halt thecompressor if the temperature falls below a lower temperature limit. 9.The refrigeration system of claim 7, further comprising an alternateenergy source, wherein the controller is configured to receive atemperature signal indicative of a temperature of the interior space ofthe enclosure, and wherein the controller is configured to enable thealternate energy source if the temperature rises above an uppertemperature limit.
 10. The refrigeration system of claim 7, wherein thecontroller is configured to calculate a voltage rate-of-changemagnitude, wherein the controller is configured to increment thecompressor speed when the voltage rate-of-change magnitude is below apredetermined threshold, and wherein the controller is configured todecrement the compressor speed when the voltage rate-of-change magnitudeis above a predetermined threshold.
 11. The refrigeration system ofclaim 1, wherein the compressor has a direct electrical connection tothe photovoltaic panel.
 12. The refrigeration system of claim 1, whereinthe compressor is a DC powered, variable speed compressor, the output ofwhich is responsive to an amount of received solar radiation.
 13. Asolar powered apparatus which comprises: a variable speed motor; a powersource which converts light into electrical power having a DC voltage; apower bus coupled to the power source and configured to provideelectrical power at said DC voltage to the variable speed motor; and acontroller coupled to the power bus to detect said DC voltage andcoupled to the variable speed motor to provide a speed control signal,wherein the controller is configured to adjust the speed control signalto maximize usage of the electrical power from the power source.
 14. Thesolar powered apparatus of claim 13, wherein the variable speed motorturns a compressor.
 15. The solar powered apparatus of claim 14, whereinthe compressor is configured to circulate refrigerant through avapor-compression refrigeration cycle to store thermal energy byinducing a phase change in a phase-change material included in a thermalreservoir.
 16. The solar powered apparatus of claim 13, wherein thecontroller determines a time derivative of said DC voltage, wherein thecontroller increases the motor speed if the derivative is above apredetermined threshold, and wherein the controller decreases the motorspeed if the derivative is below the predetermined threshold.
 17. Thesolar powered apparatus of claim 13, wherein the controller determines asecondorder time derivative of said DC voltage, and wherein thecontroller increases the motor speed if the second-order derivative isabove a predetermined threshold, and wherein the controller decreasesthe motor speed if the second-order derivative is below thepredetermined threshold.
 18. The solar powered apparatus of claim 17,further comprising a capacitor coupled in parallel with the powersource.
 19. A method for extracting maximum power from a power-limited,direct current (DC) power source having a capacitive characteristic,wherein the method comprises: measuring a power voltage; determining anunsigned rate of change for said power voltage; incrementing a rate ofpower extraction from said power source if said unsigned rate of changeis less than a predetermined threshold; and decrementing a rate of powerextraction from said power source if said unsigned rate of change isgreater than the predetermined threshold.
 20. The method of claim 19,wherein incrementing the rate of power extraction includes adjusting acompressor speed upward, and wherein decrementing the rate of powerextraction includes adjusting the compressor speed downward.
 21. Themethod of claim 20, wherein decrements to the compressor speed are atleast twice as large as increments to the compressor speed.
 22. Themethod of claim 20, wherein the compressor speed is adjusted downward byan amount proportional to the unsigned rate of change.
 23. A solarpowered compressor control method which comprises: acquiring samples ofa power voltage; determining a first unsigned rate of change for saidpower voltage; determining a subsequent unsigned rate of change for saidpower voltage; incrementing a compressor speed if said first unsignedrate of change is greater than the subsequent unsigned rate of change;and decrementing the compressor speed if said first unsigned rate ofchange is less than the subsequent unsigned rate of change.
 24. Themethod of claim 23, wherein decrements to the compressor speed are atleast twice as large as increments in the compressor speed.
 25. Themethod of claim 23, wherein the compressor speed is adjusted by anamount related to the subsequent unsigned rate of change.
 26. A solarpowered refrigeration apparatus, comprising: a compressor; a solarpowered electrical power source electrically connected to saidcompressor; a thermal energy storage device having communication withsaid compressor; and control means, associated with said power source,for accumulating electrical power derived from said power source and forconducting accumulated electrical power, and continuous electrical powerderived from said power source, to said compressor.
 27. The solarpowered refrigeration apparatus of claim 26, wherein the thermal energystorage device comprises a phase change material.
 28. A control methodfor a solar powered refrigeration system having a compressor motor,which obtains a first DC current from a solar power source, and obtainsa second DC current from a capacitance storage source, and conducts thefirst and second DC currents to said compressor motor for driving saidmotor during a “start-up” phase of said compressor motor, said methodcomprising the steps of: (A) monitoring system voltage of saidcompressor system; (B) determining whether the power required to drivesaid compressor system is greater than the power produced by said firstpower source; (C) upon determining that additional power is required tostart said compressor system, drawing additional power from saidcapacitance source for starting said compressor motor; and, (D)subsequently, driving said compressor motor substantially by said firstDC current.
 29. The control method according to claim 28, wherein step(D) comprises driving said compressor at a decreased speed, relative tonominal, during a start-up period, thereafter driving said compressor atan increased speed.
 30. A control method for a solar poweredrefrigeration system, which system obtains a first DC current from asolar power collector source, obtains a second DC current from acapacitive storage source, adds the first and second DC currents, andconducts the integrated currents to a load for operating said solarpower collector source at its maximum power point, comprising the stepsof: (A) operating said capacitance storage source as an energy buffer;(B) balancing the electrical load of said solar powered system with thepower collecting capability of said solar collector; and, (C) achievingload balance by monitoring said solar powered system voltage andadjusting the system load in relation to said voltage.
 31. The controlmethod according to claim 30, wherein step (B) comprises the steps of:(a) determining if said system load requires more power than said solarcollecting source is producing; (b) decreasing said load if saidcapacitor voltage is decreasing; (c) determining if said system loadrequires less power than said solar collecting source is producing; (d)increasing said load if said capacitor voltage is increasing; and, (e)subsequently maintaining the solar collecting source maximum operatingand collecting capacity by adjusting said voltage levels through ongoingdithering of said load.
 32. The control method of claim 31, wherein thesteps (b) and (d) are accomplished by adjusting the speed of said motor.33. A solar powered refrigeration apparatus, comprising: a vaporcompression refrigeration system; a solar powered electrical powersource electrically connected to said refrigeration system; an alternateelectrical power source connected to said refrigeration system; a devicewhich combines said power sources such that the refrigeration system canran on any combination of the two sources; and a controller whichelectrically connects the vapor compression refrigeration system to bothsources of electrical power and gives preference to the solar powersource.
 34. The apparatus of claim 33, wherein the alternate source isan alternating current (AC) power source.
 35. A solar poweredrefrigeration apparatus which comprises: a compressor system; a solarpowered electrical power source electrically connected to saidcompressor system; and a control means, associated with said solar powersource, for accumulating electrical power derived from said solar powersource, and for conducting accumulated electrical power and continuouspower derived from said solar power source to said compressor system.36. The apparatus of claim 35, wherein said control means includes acapacitive power storage means.
 37. The apparatus of claim 35, whereinsaid solar power source is directly electrically connected to saidcompressor system during start-up of said compressor system.
 38. Theapparatus of claim 35, wherein said control means furter comprises meansfor increasing a level of power applied to said compressor duringstart-up of said compressor by applying both continuous power and anincreased level of accumulated power to said compressor.
 39. Theapparatus of claim 35, wherein said control means comprises means forrepeatedly adjusting, by means of motor speed adjustments, a loadassociated with said motor to enable said motor to use substantially allof the available power.
 40. A power control system for efficientlyapplying power received from a solar collector array to an electricmotor, wherein the system comprises: buffer circuitry connected toreceive power from said solar collector array, the buffer circuitryincluding means for accumulating electrical power derived from saidsolar collector array; speed control means associated with said electricmotor; and control circuitry associated with said buffer circuitry andsaid electric motor for repetitively monitoring the motor load andavailable power available from both said solar collector array and saidbuffer circuitry, and for adjusting said speed control means to causesaid motor to utilize substantially all of the available power.
 41. Asolar powered apparatus which comprises: a variable speed motor; a powersource which converts light into electrical power having a DC voltage; apower bus coupled to the power source and configured to provideelectrical power at said DC voltage directly, without battery storagemeans, to the variable speed motor; and a controller coupled to thepower bus to detect said DC voltage and coupled to the variable speedmotor to provide a speed control signal, wherein the controller isconfigured to adjust the speed control signal to maximize usage of theelectrical power from the power source.